This invention relates to a phase change optical recording medium.
Great attention is now paid to optical recording media capable of high density recording and erasing the once recorded information for rewriting. Among such rewritable optical recording media, phase change recording media are designed such that recording is performed by irradiating a laser beam to a recording layer to change its crystalline state and reading is performed by detecting the change of reflectivity of the recording layer associated with that state change. The phase change recording media are of greater interest because the drive unit used for their operation may have a simple optical system as compared with that used for magneto-optical recording media.
For the phase change recording layer, calcogenide materials such as Gexe2x80x94Sbxe2x80x94Te are often used because of a greater difference in reflectivity between the crystalline and amorphous states and a relatively high stability in the amorphous state.
When information is recorded in a phase change optical recording medium, the recording layer is irradiated with a laser beam having a high power (recording power) sufficient to heat the recording layer at or above its melting point. In the region where the recording power is applied, the recording layer is melted and then rapidly cooled, forming a recorded mark in the amorphous state. To erase the recorded mark, the recording layer is irradiated with a laser beam having a relatively low power (erasing power) sufficient to heat the recording layer above its crystallization temperature, but below its melting point. The recorded mark to which the erasing power is applied is heated above the crystallization temperature and then slowly cooled, resuming the crystalline state. Therefore, the phase change optical recording medium allows for overwriting simply by modulating the intensity of a single laser beam.
In order to increase the recording density and transfer rate of a recording medium, attempts have been made to reduce the wavelength of recording/reading beam, to increase the numerical aperture of an objective lens in a recording/reading optical system, and to increase the linear velocity of the medium. When a recording laser beam is irradiated to a medium rotating at a linear velocity V, the recording laser beam defines on the surface of the recording layer a spot having a diameter represented by xcex/NA wherein xcex is the wavelength of the laser beam and NA is the numerical aperture of the objective lens. The spot diameter xcex/NA divided by the linear velocity V, i.e., (xcex/NA)/V gives the time of irradiation of laser beam to the recording layer, that is, the time taken for passage across a beam spot. As the recording density and transfer rate increase, the irradiation time of laser beam to the recording layer becomes shorter and shorter. This makes it difficult to optimize overwriting conditions.
Problems arising from overwriting at an increased linear velocity are discussed below.
An increased linear velocity leads to a shortened irradiation time of recording laser beam. It is then a common practice to increase the recording power in proportion to the increased linear velocity for preventing the heated temperature of the recording layer from lowering.
On the other hand, to erase the amorphous recorded mark (to recrystallize), an erasing beam must be irradiated such that the recording layer may be held for at least a predetermined time at a temperature between the crystallization temperature and the melting point. The attempt to increase the erasing power in proportion to the increased linear velocity for preventing the heated temperature of the recording layer from lowering has a less likelihood to erase the recorded mark because the irradiation time is reduced as a result of the increased linear velocity.
Therefore, to increase the linear velocity for improving the transfer rate, the recording layer must be formed of a composition having a relatively high crystal transition speed such that recrystallization is completed within a relatively short time (as disclosed in JP-A 1-78444 and JP-A 10-326436).
The recording layer featuring a high crystal transition speed, that is, a short time required for crystallization, however, is thermally less stable. That is, the recording layer suffers from the problem of low storage reliability since it readily crystallizes in a relatively hot environment.
Also, the high transfer rate can be established by such a method as by increasing the linear velocity of the medium or by increasing the linear recording density of the medium. The inventors found that reducing the recorded mark length in order to increase the linear recording density sacrifices the thermal stability of the recorded mark.
An object of the invention is to provide a phase change optical recording medium having a recording layer with an increased transfer rate and a high thermal stability.
According to the invention, there is provided an optical recording medium comprising a phase change recording layer containing antimony as a main component, the recording layer being able to be crystallized to provide a crystallized region which contains rhombohedral crystals consisting essentially of antimony and is substantially free of a crystal phase other than the rhombohedral crystals.
In one preferred embodiment, the recording layer further contains tellurium and/or indium as a main component.
In another preferred embodiment, the recording layer contains at least one element selected from the group consisting of rare earth elements, zirconium, hafnium, titanium and tin as an auxiliary component, and the medium further comprises a dielectric layer disposed contiguous to said recording layer and in front of said recording layer as viewed from the side where a recording/reading beam enters, the dielectric layer containing silicon oxide, silicon nitride, aluminum oxide, or a mixture of zinc sulfide and silicon oxide, the content of silicon oxide in the mixture being at least 30 mol %.
In the phase change recording layer containing antimony (Sb) as a main constituent component, the crystal transition speed increases as the Sb content increases. On the other hand, the thermal stability of the recording layer lowers as the Sb content increases. To improve thermal stability, it is preferable to reduce the Sb content and instead, add a thermal stability-enhancing element. This, in turn, makes it difficult to increase the crystal transition speed of the recording layer.
In the medium of the invention, the phase change recording layer provides a crystallized region which contains rhombohedral crystals consisting essentially of Sb and is substantially free of a crystal phase other than the rhombohedral crystals. If the crystalline phase included in the phase change recording layer based on Sb contains only rhombohedral crystals consisting essentially of Sb, then this recording layer provides a higher crystal transition speed than a recording layer having the same Sb content and consisting of face-centered cubic (f.c.c.) crystals and also a higher crystal transition speed than a recording layer having the same Sb content and consisting of Sb and Sb2Te3 phases. Accordingly, the invention is successful in increasing the crystal transition speed without extremely increasing the Sb content. Therefore, the invention entails a phase change optical recording medium capable of overwriting at a high linear velocity and having improved thermal stability.
In one preferred embodiment, at least one element selected from among rare earth elements, Zr, Hf, Ti and Sn is added to the recording layer as an auxiliary component, thereby elevating the crystallization temperature of the recording layer. This ensures that the recorded mark is fully thermally stable even when the recording layer is of a high crystal transition speed composition, and when the recorded mark is made short. The medium is thus improved in storage reliability.
There are known additive elements capable of elevating the crystallization temperature of a recording layer. Most additive elements, however, function to lower the crystal transition speed of the recording layer. In contrast, by adding an auxiliary component element such as rare earth element to the recording layer and at the same time, disposing a dielectric layer of a specific composition contiguous to the recording layer and in front of the recording layer as viewed from the side where a recording/reading laser beam enters the medium, the preferred embodiment of the present invention is successful in improving the thermal stability and increasing the crystal transition speed of the recording layer. The invention medium is advantageous especially in high linear velocity recording.
It is known that rare earth elements may be added to a phase change recording layer containing at least Sb. Some of the patent references to be discussed below describe that rare earth elements are effective for improving the crystallization temperature of recording layers. The patent references below, however, do not describe the recording layer containing only rhombohedral crystals consisting essentially of Sb as the crystal phase. Nor they describe the combination of the addition of rare earth elements with the dielectric layer of the specific composition prescribed by the present invention.
JP-A 2-3113 discloses an information recording thin film formed of In44Sb46Bi10 (atomic ratio) by evaporation to a thickness of 100 nm. In Examples thereof, thin films based on this composition to which Nd was added were prepared and measured for phase change temperature. A rise of phase change temperature due to Nd addition was ascertained in this way.
JP-A 2-35636 and 2-151481 disclose phase change information recording thin films containing Sb, Te and other elements. It is described that the crystallization temperature can be elevated by adding rare earth elements or analogues. In Examples thereof, however, no thin films having rare earth elements added were prepared.
JP-A 10-326436 describes that rare earth elements may be added to a phase change recording layer containing Sb and Te. It is described that Zn, Cu, Au, Ag, Pd, Pt, Cr, Co, Zr, Ti, Mn, Mo, Rh and rare earth elements themselves or compounds thereof with Sb or Te have high melting points so that they precipitate as fine dispersed clusters and serve as crystal nuclei, contributing to high speed crystallization. Examples therein lack samples having rare earth elements added. Recording at a wavelength 780 nm, a numerical aperture NA of 0.55 and a linear velocity of 4.8 m/s or below is described in Examples, which indicates that high linear velocity recording as contemplated in the present invention is out of consideration.
JP-A 2000-43415 discloses a phase change recording layer containing Sb and Te and having a metastable Sb3Te phase belonging to the space group Fm3m. This Sb3Te phase has a face-centered cubic (f.c.c.) structure as described therein. It is also described that by adding nitrogen, boron, carbon, rare earth element or transition metal element to the recording layer, an improvement is made in the long-term storage of initially recorded data. However, Examples therein lack samples having rare earth elements added. It is also noted that in Examples, the linear velocity for recording is 7 m/s.
JP-A 2000-52657 discloses a phase change recording layer containing Sb, Te, Group Ib element and Group IIIb element and having a metastable phase belonging to the space group Fm3m. This metastable phase is a f.c.c. structure phase as typified by Sb3Te phase. Although this patent describes that rare earth elements may be added to the recording layer, Examples therein lack samples having rare earth elements added. In Examples, the linear velocity for recording is 8 m/s or below. It is also described that the content of Ag+Au in the recording layer is adjusted in accordance with the linear velocity. Namely, in this patent, compositions free of Ag and/or Au are out of consideration. More specifically, Table 2 in this patent publication describes a recording layer 14 of Ag0.05 In0.04Sb0.61Te0.3 containing a f.c.c. phase, Sb2Te3 phase and Sb phase. That is, this recording layer is a mixture of a f.c.c. phase and a rhombohedral crystal phase. It is described that in evaluating the recording layer 14, Sb and Sb2Te3xe2x80x94which develop as precipitates when the metastable phase is differentiatedxe2x80x94precipitate in the recording layer, which is inadequate for high density recording.
JP-A 9-71049 discloses an optical information recording medium having a recording layer consisting of Sb, Te and M wherein M is at least one element of Ag, Cu and Au, wherein these constituent elements are present as SbxTe1-x wherein 0.70 less than x less than 0.90 and (MzTe1-z)bSb wherein 0 less than z less than 0.33 and 0 less than b less than 1. It is described in paragraph 0013 of the patent publication that on X-ray analysis, a sharp peak of Sb rather than that of Sb2Te3 appears. That is, this recording layer contains both a Sb2Te3 phase and a Sb phase as crystals, but not a Sb phase alone. Therefore, the recording layer of this patent publication differs from the recording layer of the present invention and fails to achieve the desired effects of the present invention. It is not described in this patent publication that auxiliary component elements as prescribed in the present invention are added to the recording layer.